Color contamination can occur in the manufacture of low molecular weight cellulose ethers. The contamination takes the form of carbonacious tars which are brown or black in color. The tars take the form of dark particulates in powdered cellulose ether end products. The presence of the dark particulates is problematic if the cellulose ether products are to be used in appearance-sensitive applications such as tablet coatings for pharmaceuticals. Particulate contamination in tablet coatings is unacceptable to consumers and manufacturers.
Carbonacious tars form as a result of excessive degradation during depolymerization of high molecular weight cellulose ethers to low molecular weight cellulose ethers. Depolymerization is typically effected by addition of a strong acid, i.e. hydrogen chloride, to high molecular weight cellulose ethers to cleave a fraction of the ether linkages.
Excessive degradation of cellulose ethers during depolymerization can result from the following: non-uniform moisture distribution within the cellulose ether; static cling between the cellulose ether and the internal surfaces of the depolymerization reactor; non-uniform application or adsorption of the strong acid; and catalyzation of the depolymerization reaction due to contact with catalytically active surfaces or substances.
Excessive degradation due to non-uniform moisture distribution occurs when water absorbed within the cellulose ether vaporizes and condenses on colder spots on the internal surfaces of the depolymerization reactor. Dry cellulose ether powder absorbs the condensed water and sticks to the internal surfaces. The strong acid, i.e. hydrogen chloride, is also attracted to the high moisture resulting in elevated acid content at points on the internal surfaces. The elevated acid content causes the depolymerization rate to increase severalfold compared to the bulk of the cellulose ether powder. The elevated reaction rate causes tar particulate formation at points on the internal surfaces of the depolymerization reactor.
Excessive degradation due to static cling occurs when a film of cellulose ether powder clings to stagnant regions of the internal surfaces of the depolymerization reactor. When the reactor is emptied, the powder film clings to the surfaces and is subject to overreaction from exposure to subsequent batch reactions. This overreaction can result in tar particulate formation at regions on the internal surfaces of the depolymerization reactor.
Excessive degradation can occur from the non-uniform application or absorption of the depolymerizing acid when it is introduced into the reactor in a non-gaseous form such as an aqueous liquid. The non-uniform application or absorption results in elevated acid content in portions of the cellulose ether powder. The elevated acid content causes the depolymerization rate to increase severalfold compared to the bulk of the cellulose ether powder. The elevated reaction rate can cause tar particulate formation within the cellulose ether powder.
Excessive degradation can occur from catalyzation of the depolymerization reaction by contact with catalytically active surfaces or substances. If internal surfaces of the reactor are catalytically active, the elevated reaction rate can result in tar particulate formation at those surfaces.
It would be desirable to have a process and apparatus for making low molecular weight cellulose ethers with a substantially reduced incidence of tar formation.